The present invention relates to a pressure-sensitive adhesive which is used for various labels, tapes, sheets and the like, and is excellent in quality stability, processability and storage stability, and pressure-sensitive adhesive properties such as adhesion, holding power and bonding power to a curved surface; a pressure-sensitive adhesive product; and a process for producing the same.
Hitherto, as a pressure-sensitive adhesive, acrylic or rubber type polymer has been used. The pressure-sensitive adhesive has, however, the following problems. Firstly, if thickness of the pressure-sensitive adhesive is not strictly controlled, at the time of coating the pressure-sensitive adhesive to a backing in the production step, the resultant products come to have very different adhesions. In the case of any acrylic pressure-sensitive adhesive, the thickness of the coated pressure-sensitive adhesive is usually controlled at about 20-25 g/m2. The pressure-sensitive adhesive has a property that the larger the coated thickness becomes, the larger the adhesion becomes; and the smaller the coated thickness becomes, the smaller the adhesion becomes. For this reason, common knowledge in the prior art is that if the coated thickness varies, the adhesion of the products also varies. In order to prevent troubles occurring particularly by the fact that the coated thickness inclines to become thin, coating-makers adapt so that the pressure sensitive adhesive is coated so as to become thick by 20-30% (25-30 g/m2) in the present circumstances.
However, if the layer of the pressure-sensitive adhesive becomes thick in such a way, costs rise up and productivity falls. Moreover, in various steps subsequent to the coating, the following problems occur.
Namely, a pressure-sensitive adhesive sheet is usually slit by using a rotating sharp-edged tool to cut the raw roll thereof coated with a pressure-sensitive adhesive into an appropriate width, and the sheet is die cut into an appropriate size at the time of printing. In these slit-or die-cutting procedures, the pressure-sensitive adhesive remains or adheres on the edged tool, or the pressure-sensitive adhesive spreads out and is not cut. Thus, the pressure-sensitive adhesive transfers to the product or other parts, so that the product gets dirty or the sheet is not satisfactorily cut. Such various troubles arise.
And then, concerning a tape or sheet raw-roll, in the step of storing the products, a pressure sensitive adhesive is oozed from its edge-face by its weight and/or rolling-tightening pressure. Such phenomena arise, too. Besides, concerning raw-roll products die-cut into a label form after printing, their pressure-sensitive adhesive is oozed from their backing in the same manner so that the raw-rolls adhere to each other or the labels themselves shift on release paper and get out of their proper-position. In any case, the pressure-sensitive adhesive transfers onto an actual item for sale during the step of sticking the label, so that the item itself gets dirty. During the process of affixing the labels with a machine, the adhesive tends to shift the label out of proper-position. Such troubles tend to arise more frequently in the case that the backing is a plastic film than in the case that the backing is paper.
When a pressure-sensitive adhesive is coated to have a large thickness in this way, various troubles are caused. Therefore, coating-makers take, as an adoption, that a hardening agent (crosslinking agent) is added in a larger amount to, e.g., a solvent-type pressure-sensitive adhesive so that the pressure-sensitive adhesive is made hard and its cohesion is made high. According to the method, however, its original and high adhesion is suppressed so that the real performances of pressure-sensitive adhesive are not exhibited. As a result, there arises a dilemma that the pressure-sensitive adhesive should be coated thicker and thicker.
As for an emulsion-type pressure-sensitive adhesive, holding power can be obtained if a cohesive component is added thereto. There is, however, caused a phenomenon that its adhesion is extremely lowered.
Many emulsion pressure-sensitive adhesives are produced by emulsifying-polymerizing (meth)acrylate such as butyl acrylate and 2-ethylhexyl acrylate. However, a pressure-sensitive adhesive using, as a monomer, only (meth)acrylate has insufficient holding power necessary after that a tape or the like coated with the pressure-sensitive adhesive is stuck on an adherent.
Thus, the method, which aims to raise the cohesive power of the emulsion and which improves the holding power of the emulsion, is conducted by adding a polar monomer, such as acrylic acid or adding a crosslinking monomer having two or more reactive groups, such as divinylbenzene, to (meth)acrylate and copolymerizing them. The addition of the polar monomer, however, causes a fall in the water-resistance of the pressure-sensitive adhesive, thereby resulting in such a drawback that the adhesion onto any non-polar adherent such as polyolefin is largely lowered. Moreover, when a cohesive component such as these polar monomers and these crosslinking agents is added to the pressure-sensitive adhesive, the pressure-sensitive adhesive becomes elastic so that adhesion and tackiness are largely lowered.
As the above-mentioned, conventionally, it is difficult to produce an emulsion-type pressure-sensitive adhesive having good balance of pressure-sensitive adhesive performances so as to satisfy sufficiently both properties of holding power (cohesion) and adhesion.
Besides, it is important to have sufficient bonding power to a curved surface as one of the properties of a pressure-sensitive adhesive. In the case that an adherent has a curved surface, the larger the curvature thereof is, the more easily a pressure-sensitive adhesive sheet is stripped from its edge face. In other words, there arises a so-called edge lifting phenomenon that the force of repulsion of the backing against the pressure-sensitive adhesive causes stress for stripping the pressure-sensitive adhesive sheet from the curved surface of the adherent so that the edges of the pressure-sensitive adhesive sheet are gradually lifted up with the passage of time. In the present situation, there is not known a clear theory on such bonding power to a curved surface. A trial-and-error method has been repeated in the industry. The bonding power to a curved surface, which may be referred to as edge lifting resistance, is one of the important performances required for pressure-sensitive adhesives. It should be not lacking for the quality of a final product being stuck with such a pressure-sensitive adhesive label or sheet to satisfy this bonding power sufficiently. In the present situation, however, there is insufficient bonding power to a curved surfaces by various kinds of adherents, in particular an adherent made of polyolefin type compound.
Then, JP-A 7-330813 and WO097/07174 disclose an emulsion for a pressure-sensitive adhesive, comprising 50% or more by weight of a long-chain alkyl (meth)acrylate. However, the emulsion cannot sufficiently meet the above-mentioned demands.
The inventors have found that the above-mentioned object can be attained by a pressure-sensitive adhesive exhibiting a specific storage modulus in dynamic viscoelasticity measurement and having a specific gel fraction. The inventors have found that the above-mentioned pressure-sensitive adhesive can be produced by a process for producing an emulsion-type pressure-sensitive adhesive in which the manner of adding a polymerization-initiator is devised.
The present invention is composed of the pressure-sensitive adhesive whose adhesion does not fall even when it is thin coated, and the present invention provides the excellent pressure-sensitive adhesive which can solve fluctuation in quality based on fluctuation in the coated thickness of the pressure-sensitive adhesive, which is excellent in slitting property and property for die-cutting into a label, a tape, a sticker or the like, and which is not oozed from its backing even at the time of being stored at high temperature in summer; and provides a pressure-sensitive adhesive product thereof.
The present invention also relates to the emulsion-type pressure-sensitive adhesive having a good balance of pressure-sensitive adhesive performances, that is, having both of high adhesion and high holding power, which are usually difficult to be compatible with each other, and exhibiting excellent bonding power to a curved surface; and relates to the process for producing the pressure-sensitive adhesive.
The pressure-sensitive adhesive of the present invention is excellent in holding power and adhesion, and has the balance of pressure-sensitive adhesive performances; and can be effectively used in various applications.
The pressure-sensitive adhesive has high adhesion performances even if the coated thickness thereof is thin, and is excellent in holding power. There are produced particles having crosslinked structure wherein its inner portion is soft but its surface is hard by multi-step polymerization in which a water-soluble polymerizing-initiator is added during or after the second step. That is, the balance of adhesion and holding power is good.
In dynamic viscoelasticity measurement, the storage modulus, measured at 25xc2x0 C. and at a frequency of 1 Hz, of the pressure-sensitive adhesive of the present invention ranges preferably from 1xc3x97105-1xc3x97106 dyne/cm2, and more preferably from 1xc3x97105-6xc3x97105 dyne/cm2. The gel fraction in the pressure-sensitive adhesive is preferably 55% or more, and more preferably 60% or more.
Herein, the storage modulus can be measured by applying shear stress at a given frequency to a pressure-sensitive adhesive layer coated in a predetermined step and, if desired, heating the pressure-sensitive adhesive.
In many cases, conventional pressure-sensitive adhesives have a storage modulus of 1xc3x97106-1xc3x97107 dyne/cm2, but the inventors have found that the pressure-sensitive adhesive of the present invention has a storage modulus of about 1xc3x97105-about 1xc3x97106 dyne/cm2, which is one-digit smaller than the conventional pressure-sensitive adhesives, so as to give appropriate pressure-sensitive adhesive properties.
Even if the storage modulus is 1xc3x97106 dyne/cm2 or less, which is smaller than conventional ones, the pressure-sensitive adhesive does not become hard so as to give original adhesion of the pressure-sensitive adhesive, which is a target of the present invention.
Then, if the storage modulus is 1xc3x97105 dyne/cm2 or more, the pressure-sensitive adhesive itself does not become excessively soft so that cohesion destruction does not occur at the time of peeling. Holding power is also improved, so that any trouble does not arise at the time of processing such as die-cutting of a pressure-sensitive adhesive sheet or at the time of storage.
Necessarily, while the storage modulus is kept within the above-mentioned range, the gel fraction in the present invention is essentially 55% or more, and preferably 60% or more. Holding power is raised by setting the gel fraction to 55% or more, so that the pressure-sensitive adhesive attains its original action.
Herein, the gel fraction means a proportion of the pressure-sensitive adhesive that is not dissolved when the adhesion is incorporated into a given solvent for the purpose of dissolution.
In the case that the gel fraction of conventional pressure-sensitive adhesives is raised to make its cohesion large, the pressure-sensitive adhesives themselves get hard so that its storage modulus is also raised. It is difficult that, for example, the storage modulus is kept below 1xc3x97106 dyne/cm2 while the gel fraction is kept above 55%. According to the present invention, in an example of the process for producing the pressure-sensitive adhesive, which will be described later, the above-mentioned properties can have been satisfied. In other words, high holding power can be obtained while high adhesion can be kept. This makes it possible to satisfy properties as a pressure-sensitive adhesive and supply strong adhesion, even if the pressure-sensitive adhesive is thin coated, so as to give such properties as good processability and good storage stability.
In the present invention, when the ratio, at the temperature of 25 xc2x0 C., of the storage modulus measured at a frequency of 100 Hz to the storage modulus measured at a frequency of 1 Hz satisfies the following formula (1), high bonding power to a curved surface can be obtained.
5 less than storage modulus (100 Hz)/storage modulus (1 Hz) less than 30xe2x80x83xe2x80x83(1)
The boding power to a curved surface means resistance against the phenomenon (edge lift) that when a pressure-sensitive adhesive sheet product such as a label is stuck onto a curved surface of an adherent, the edges of the sheet are lifted up with the passage of time.
For evaluation, a pressure-sensitive adhesive sheet is stuck onto each of columnar rods, which have Ø of 10 to 13 mm and is made of various adherent materials, and then the state that its edge is lifted up is observed for judgement. The evaluation is usually performed by using a pressure-sensitive adhesive sheet wherein a pressure-sensitive adhesive is coated in an amount of about 20 g/m2 to a film made of polyethylene terephthalate having a thickness of 50 xcexcm.
In the present invention, it is unclear why the bonding power to a curved surface is excellent if the ratio of the storage modulus measured at a frequency of 100 Hz to the storage modulus measured at a frequency of 1 Hz is within the above-mentioned range. The bonding power to a curved surface is a property in the static state, and is a factor for evaluating the state that the edge of the pressure-sensitive adhesive sheet is gradually lifted up by static repulsion force generated when the backing of the pressure-sensitive adhesive sheet stuck onto the curved surface of the adherent is returned to the original state. Herein, the lower the storage modulus is, the softer the pressure-sensitive adhesive gets. Furthermore, a capability of following roughness in the surface of the adherent gets good. As a result, the adhesion of the pressure-sensitive adhesive would be raised. Besides, the pressure-sensitive adhesive is transformed to follow fine deformation that causes lifting-up by the repulsion force of backing. Thus, even if a little deformation arises, the interface between the pressure-sensitive adhesive and the backing or the adherent is not destructed so that the bonding power to the curved surface would be made good.
Although the storage modulus in the static state cannot be directly measured, it appears that the modulus can be presumed by extrapolating a measured dynamic viscoelasticity value toward the side of lower frequencies. That is, it is presumed that the larger the storage modulus measured at a frequency of 100 Hz compared with the storage modulus measured at a frequency of 1 Hz is (i.e., the larger the ratio (or inclination) of the storage modulus (100 Hz)/ the storage modulus (1 Hz) is), the smaller the storage modulus at the side of lower frequencies becomes. It appears that the adhesion in the static state gets larger so that resistance against edge-lifting based on that the repulsion force of backing becomes larger.
When the repulsion force of backing becomes larger, it can be presumed that the larger the value (or ratio) of the storage modulus (100 Hz)/ the storage modulus (1 Hz) is, the better the bonding power to a curved surface is. However, in reality, when the ratio exceeds 30, the adhesion at the time of release becomes small and/or tackiness falls, etc. The pressure-sensitive adhesive comes not to attain its original function.
When the pressure-sensitive adhesive satisfies the above-mentioned properties, the process for producing thereof is not restrictive, but the following process is preferable, for example.
That is; the process of adding, when alkyl (meth)acrylate is polymerized in the presence of polymerization-initiator(s), the polymerization-initiator(s) at separate multi-steps into its reaction system, and using a water-soluble polymerization-initiator at least one time during or after the second step for adding the polymerization-initiator(s).
Examples of the process for preparing (meth)acrylic emulsion include emulsion polymerization, pre-emulsion polymerization and suspension polymerization. In order to prepare the emulsion of the present invention, any one of these processes may be used.
In the present invention, the polymerization-initiator that is first added (i.e., at the first addition) may be water-soluble or oil-soluble. During or after the second step, however, a water-soluble polymerization-initiator needs to be added at least one time. In this case, the following is thought: the addition of the water-soluble polymerization-initiator causes crosslinking reaction based on pulling-out of hydrogen of acrylate or the like near the surface of emulsion particles, so as to raise the gel fraction; therefore, cohesion acts as a whole to improve the holding power of the emulsion. The polymerization-initiator acts only near the surface of the emulsion because the initiator is water-soluble. Thus, crosslinking is not caused inside the emulsion so that the emulsion is soft and the pressure-sensitive adhesive is not hard as a whole. Thus, the adhesion thereof is kept good.
From the above-mentioned standpoint, it is preferred that the water-soluble polymerization-initiator is added after reaction of the monomer advances to some degree and the particle size of the resultant is made stable. In the case that the water-soluble polymerization-initiator is used in the first step, polymerization reaction takes priority over crosslinking reaction because of a large amount of the monomer in the system so that the effect of improving cohesion is small. By adding the water-soluble polymerization-initiator during or after the second step, the gel fraction rises in the emulsion and further its molecular weight distribution widens from a low molecular weight to a high molecular weight. This fact appears to contribute to the improvement in holding power and adhesion.
It is allowable that the oil-soluble polymerization-initiator is first added and then the water-soluble polymerization-initiator is added to perform polymerization step by step.
Examples of the water-soluble polymerization-initiator include peroxide type polymerization-initiators such as t-butylhydroperoxide, potassium peroxodisulfate, ammonium peroxodisulfate; azo type polymerization-initiators; and redox polymerization-initiators, which are obtained by combining any one of these initiators with a reductant such as ascorbic acid, sodium bisulfite or iron ion. These may be used alone or in combination. As the manner of adding the polymerization-initiator in the respective steps, any manner of adding at a time, adding dropwise, and continuous adding may be used. Preferably, the water-soluble polymerization-initiator added during or after the second step, among all of the added polymerization-initiators, is a substance which is dissolved in an amount of 0.1% by weight or more, and in particularly 1% by weight or more, at the reaction temperature in water. It is preferred from the standpoint of adhesion that the amount thereof is from 0.1 to 1.0% by weight and in particular from 0.1 to 0.5% by weight of the whole charged amount.
The adding cycle (interval of additions) of the polymerization-initiators, including the first adding step, and the added amounts of the respective steps are not especially limited, and may be appropriately decided considering the initial added monomer, the polymerization manner or the like.
According to the above-mentioned process, even in alkyl (meth) acrylate having a relatively short alkyl chain, for example, butyl acrylate or 2-ethylhexyl acrylate, which has been conventionally used as alkyl (meth) acrylate for emulsion-type pressure-sensitive adhesives, its holding power and its adhesion are compatible with each other to exhibit sufficient effect. Further, in the case of using a long chain alkyl (meth) acrylate having 9 or more carbon atoms as the monomer, the addition of a small amount of the crosslinking agent causes more reduction in adhesion than in the case of using one having a short alkyl chain. For this reason, it is more effective to add the water-soluble polymerization-initiator during or after the second step.
It is preferred to obtain the (meth)acrylic emulsion by polymerizing alkyl (meth)acrylate mainly containing a long chain alkyl (meth)acrylate having a C9-14 alkyl group. It is preferred to use, for example, 70% by weight or more of such a long chain alkyl (meth)acrylate which has an alkyl group having 9 to 14 carbon atoms, as a main component. Specific examples thereof include nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, dodecyl (meth)acrylate, isododecyl (meth)acrylate, tridecyl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate and isotetradecyl (meth)acrylate. These may be used alone or in combination, or may be used with other polymerizable monomer(s).
The emulsifier used in the polymerization is not limited. However, in the case that a radical polymerizable emulsifier is used, it is more effective to add the water-soluble polymerization-initiator during or after the second step.
The radical polymerizable emulsifier/dispersant is an emulsifier having a radical polymerizable double bonding group in its molecule. An appropriate emulsifier can be selected from the standpoint of copolymerizability with the monomer, monomer-emulsifying action and dispersion stability of polymer particles, regardless of the structure of its hydrophilic group, such as a nonionic, cationic and anionic structure. The polymerizable emulsifier/dispersant is a surfactant having an allyl group, (meth)acrylate, styrene group and/or isopropenyl group as polymerizable group in its molecule. The number of carbon atoms in its hydrophobic group is preferably from 8 to 20. For example, JP-A 53-126093, JP-A 56-28208, JP-A 4-50204, JP-A 62-104802, JP-A 50-98484, JP-A 54-144317, JP-A 55-115419, JP-A 62-34947, JP-B 49-46291, JP-A 58-203960, JP-A 4-53802, JP-A 62-104802, JP-A 49-40388 and JP-A 52-134658 can be referred. These radical polymerizable emulsifiers/dispersants are used so far as the advantage of the present invention is not damaged, and are usually used in the range of 0.1 to 2.0 parts by weight per 100 parts weight of all of the monomer component.
If necessary, a tackifier or a thickener may be added. As the tackifier, any one among rosins, rosin derivatives, petroleum resins, terpene resins or the like may be used. The tackifier is usually used preferably in the range of 0.1 to 30 parts by weight per 100 parts weight of the polymer component in the emulsion. Examples of a nonionic thickener include hydroxyethylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol and alkyl-modified compounds thereof. Above all, the thickener having an alkyl group, particularly alkyl-modified polyvinyl alcohol, is preferable, since it can freely cause a change in the number of carbon atoms of the alkyl group, saponification value and the like and it can easily control thickness. Usually, the nonionic thickener is preferably used in the range of 0.1 to 5.0 parts by weight per 100 parts weight of the polymer component in the emulsion. Further, it is allowable to blend the other component such as an additive such as a pH adjuster, a defoaming agent, a preservative or a pigment so far as it does not cause a fall in the pressure-sensitive adhesive performance of the emulsion-type pressure-sensitive adhesive.
In a pressure-sensitive adhesive product, the amount of its pressure-sensitive adhesive layer is preferably from 3 to 15 kg/m2. The pressure-sensitive adhesive product can be produced by stacking a release sheet, the pressure-sensitive adhesive layer and backing in this order. A mono-web type pressure-sensitive adhesive sheet may be obtained by applying a release agent to the surface of the backing and then applying the pressure-sensitive adhesive to the back surface thereof.
The emulsion-type pressure-sensitive adhesive obtained by the producing process of the present invention can be produced into a pressure-sensitive adhesive product such as a pressure-sensitive adhesive sheet, tape and label by firstly directly applying the pressure-sensitive adhesive to a plastic backing, a paper backing or the like with e.g., a comma coater or a gravure coater and then drying the pressure-sensitive adhesive, or by firstly applying the pressure-sensitive adhesive to a releasing substrate, and then drying the pressure-sensitive adhesive, and secondly laminating the pressure-sensitive adhesive with a plastic backing, a paper backing or the like and transferring the pressure-sensitive adhesive. These pressure-sensitive adhesive products are excellent in quality stability * processability * storage stability, and further pressure-sensitive adhesive properties such as adhesion, holding power and bonding power to a curved surface. Thus, the pressure-sensitive adhesive product can be effectively used in various applications.